High purity carbonaceous material and ceramic coated high purity carbonaceous material

ABSTRACT

The invention provides a high purity carbonaceous material which is reduced in contents of oxygen, nitrogen and chlorine readily binding to carbon atoms and in contents of elements, phosphorus, sulfur and boron, readily binding to carbon atoms upon heating and which can be used in producing single crystals such as semiconductors, a high purity carbonaceous material for use as a substrate for ceramic layer coating, and a ceramic layer-coated high purity carbonaceous material. The high purity carbonaceous material has oxygen content of 1×10 18  atoms/cm 3  or less as determined by SIMS. Its chlorine content is preferably 1×10 16  atoms/cm 3  or less as determined by SIMS, and its nitrogen content is preferably 5×10 18  atoms/cm 3  or less as determined by SIMS. Its phosphorus, sulfur and boron contents are preferably not higher than respective specified values. Such a high purity carbonaceous material is coated with ceramic layer.

FIELD OF THE INVENTION

This invention relates to a carbonaceous material with a very lowimpurity content and, more particularly, to a high purity carbonaceousmaterial suited for use in the semiconductor industry in producingsilicon single crystals, silicon carbide (SiC) single crystals, galliumnitride (GaN), calcium fluoride (CaF₂) and so forth or in the nuclearindustry and for use as a substrate for ceramic layer coating. It alsorelates to a ceramic coated high purity carbonaceous material comprisingsuch high purity carbonaceous material as a substrate.

BACKGROUND OF THE INVENTION

Carbonaceous materials are not only excellent in various mechanicalcharacteristics but also advantageous in that they hardly react withmetals. Therefore, they are widely used in the semiconductor, machineryand nuclear industries.

Recent years, market of silicon semiconductors and compoundsemiconductors, typically SiC and GaN are rapidly expanded. With suchmarket expansion, the requirements imposed on carbonaceous materialshave become severer. In addition, carbonaceous materials are also usedin the growth of CaF₂ single crystals, which are used for emitting shortwavelength excimer laser beams in semiconductor lithography (hereinafterreferred to as “photo etching”) for attaining large scale integration ofsemiconductor.

Enhanced resolution is required in semiconductor photo etching and, forrealizing resolution enhancement, CaF₂ has come into use for emittingshort wavelength excimer laser beams, such as krypton fluoride (248 nm),argon fluoride line (193 nm) and fluorine (157 nm). In connection withthis, lenses made of fluorite (single crystal CaF₂) have come into use,since the conventional noncrystalline optical materials cannot transmitlight at 193 nm. In the following, the production of CaF₂ singlecrystals is described specifically. CaF₂ single crystals are produced bythe Bridgman technique or Czochralski (CZ) process. The use of agraphite material as jigs in furnace, such as heating elements, in theproduction of CaF₂ single crystals by the Bridgman technique, forinstance, is described in Laid-open Japanese Patent Application (JPKokai) No. 2000-137101.

Graphite materials generally contain metal impurities entrapped in poresand between graphite lattices thereof, hence they as such cannot be usedin semiconductor manufacturing. Therefore, the applicants havepreviously proposed, for use in the semiconductor and nuclearindustries, high purity graphite materials having a metal impurity (ash)content of 5 ppm or less as a result of treatment of graphite materialswith a halogen-containing gas, for instance, for attaining high levelsof purity (JP Kokai No. S64(1989)-18964; Japanese Patent Publication (JPKokoku) No. H06(1994)-35325). They have also recently proposed, in JPKokai 2002-249376, carbonaceous materials with reduced nitrogen contentfor use in the manufacture of compound semiconductors.

However, even if such high purity graphite materials reduced in metalimpurity content and in nitrogen content as disclosed in the above-citedpatent documents are used as jigs in furnace, the yield in theproduction of CaF₂ single crystals becomes very low, less than 10%,because impurities, such as oxygen, chlorine, phosphorus and sulfur,work badly.

An object of the present invention is to provide the high puritycarbonaceous material, which reduced not only oxygen, nitrogen,chlorine, phosphorus and sulfur in pores but also oxygen, nitrogen,chlorine, phosphorus, sulfur and boron which are bound to carbon atomsin graphite material.

SUMMARY OF THE INVENTION

As a result of intensive investigations made by them to accomplish theabove object, the present inventors could find those purificationtreatment conditions which are suited for the removal of theabove-enumerated impurity elements, namely the reductions in contents ofcarbon atom-bound oxygen, nitrogen, chlorine, phosphorus, sulfur andboron bound to carbon atoms. Based on such and other findings, they havenow completed the present invention.

The present invention provides a high purity carbonaceous materialhaving oxygen content of 1×10¹⁸ atoms/cm³ or less analyzed by SIMS(secondary ion mass spectrometry). For instance, it is necessary toreduce the oxygen concentration to a level as low as possible inproducing silicon carbide single crystals. The use of a carbonaceousmaterial with an oxygen content of 1×10¹⁸ atoms/cm³ or less makes itpossible to obtain single crystals having good semiconductorcharacteristics. More preferably, the oxygen content is 3×10¹⁷ atoms/cm³or less, most preferably 1×10¹⁷ atoms/cm³ or less.

In another aspect, the invention provides a high purity carbonaceousmaterial having a chlorine content of 1×10¹⁶ atoms/cm³ or less analyzedby SIMS. When the carbonaceous material used as furnace jigs inepitaxial growth of SiC has a chlorine concentration of 5×10¹⁵ atoms/cm³or less, it becomes possible to markedly decrease the chlorine in theepitaxial growth layer. Thus, the chlorine content is more preferably8×10¹⁵ atoms/cm³ or less, most preferably 5×10¹⁵ atoms/cm³ or less.

In a further aspect, the invention provides a high purity carbonaceousmaterial having a nitrogen content of 5×10¹⁸ atoms/cm³ or less asmeasured by SIMS. In producing silicon carbide single crystals, it isnecessary to reduce the nitrogen concentration, which is the mainimpurity, as far as possible. The use of a carbonaceous material with anitrogen content of 5×10¹⁸ atoms/cm³ or less makes it possible tomarkedly reduce the nitrogen concentration in SiC single crystals. Morepreferably, the nitrogen content is 5×10¹⁷ atoms/cm³ or less, mostpreferably 5×10¹⁶ atoms/cm³ or less.

In a further aspect, the invention provides a high purity carbonaceousmaterial having a phosphorus content of 1×10¹⁶ atoms/cm³ or less asmeasured by SIMS. The use of a carbonaceous material with a phosphoruscontent of 1×10¹⁶ atoms/cm³ or less as jigs for the production of SiCsingle crystals makes it possible to markedly reduce the phosphorusconcentration in the single crystals. More preferably, the phosphoruscontent is 3×10¹⁵ atoms/cm³ or less, most preferably 1×10¹⁵ atoms/cm³ orless.

In a further aspect, the invention provides a high purity carbonaceousmaterial having a sulfur content of 1×10¹⁶ atoms/cm³ or less as measuredby SIMS. When CaF₂ single crystals are produced using a carbonaceousmaterials having a sulfur content of 1×10¹⁶ atoms/cm³ or less as heatingelements, it becomes possible to markedly improve the transmissivitythereof More preferably, the sulfur content is 5×10¹⁵ atoms/cm³ or less,most preferably 3×10¹⁵ atoms/cm³ or less.

In a still further aspect, the invention provides a high puritycarbonaceous material having a boron content of 5×10¹⁶ atoms/cm³ or lessas measured by SIMS. Boron is one of the major impurities in theproduction of SiC semiconductors. When production jigs made of acarbonaceous material with a boron concentration of 5×10¹⁶ atoms/cm³ orless, are used for manufacturing semiconductors, SiC single crystalswith low boron concentration and excellent semiconductor characteristicscan be produced. More preferably, the boron content is 1×10¹⁶ atoms/cm³or less, most preferably 5×10¹⁵ atoms/cm³ or less.

In this way, the invention can provide the extremely high puritycarbonaceous material, which reduced not only oxygen, nitrogen,chlorine, phosphorus and sulfur contained in pores but also oxygen,nitrogen, chlorine, phosphorus, sulfur and boron readily bound to carbonatoms in carbon materials.

The high purity carbonaceous material of the invention can preventcrystal defects during the manufacturing of SiC single crystals, siliconsingle crystals, GaN single crystals or CaF₂ single crystals, amongothers, and therefore can be adequately used in the production of suchcrystals. In addition, it can be used as the material of jigs for theepitaxial growth of SiC, GaN, silicon, etc..

Furthermore, the high purity carbonaceous material of the invention canbe used as a substrate material for ceramic coatings such as SiC, boronnitride, tantalum carbide and so on.

When the above high purity carbonaceous material is used as a substratematerial for ceramic coatings and the surface thereof is coated withceramic such as SiC, boron nitride or tantalum carbide, ceramiclayer-coated high purity carbonaceous materials with extremely lowimpurity concentrations can be produced.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flowchart illustrating a process for producing the highpurity carbonaceous material of the invention.

DETAILED DESCRIPTION OF THE INVENTION

First, the high purity carbonaceous material of the invention isdescribed in detail.

The high purity carbonaceous material of the invention is derived from amaterial, which is extremely purified carbon material defined in theconventional meaning. Thus, it is derived, by treatment forpurification, from (1) a calcined carbonaceous material prepared bymixing up one or more of finely divided natural graphite, artificialgraphite, petroleum coke, coal coke, pitch coke, carbon black andmesocarbon, and a binder such as pitch, coal tar, coal tar pitch or athermosetting resin, followed by kneading, pulverizing, molding andcalcining, or a graphitized carbonaceous material derived therefrom byfurther graphitizing according to need, (2) a noncrystalline (glassy)carbonaceous material prepared by carbonization of a thermosetting resinsuch as a phenol resin, (3) a carbon fiber-reinforced, carbon-basedcomposite material produced by the resin char process comprisingrepetitions of a procedure consisting of coating/impregnation of one ofvarious carbon fibers such as polyacrylonitrile (PAN)-, pitch- orrayon-derived ones with a binder selected from among pitch, phenolresins and other ones mentioned above, molding, calcining and resinimpregnation, or a carbon fiber-reinforced, carbon-based compositematerial resulting from impregnation or coating with pyrolytic carbon inlieu of the resin, or (4) a sheet form of graphite prepared by 10- toseveral hundred-fold expansion of natural or artificial graphite,followed by compression molding, for instance.

A process for producing the high purity carbonaceous material of theinvention is now described.

A flowchart illustrating the process for producing the high puritycarbonaceous material of the invention is shown in FIG. 1.

The process for producing the high purity carbonaceous material of theinvention comprises the process of purity improvement by treatment in agaseous atmosphere of halogen or compound thereof, for example chlorine,trichloromethane, dichloromethane, monochloromethane, fluorine,trifluoromethane, difluoromethane, monofluoromethane,monochlorotrifluoromethane, dichlorofluoromethane, trichlorofluoromethane, monochloroethane, monochlorofluoroethane, monochlorodifluoroethane,monochlorotrifluoroethane, dichloroethane, dichloromonofluoroethane,dichlorodifluoroethane, dichlorotrifluoroethane, trichloroethane,trichloromonofluoroethane, trichlorodifluoroethane or tetrachloroethane,at the temperature of 2400° C. or higher (preferably 2450° C. or higher)to thereby eliminate such impurity metals as boron (B) and vanadium (V)(high purification process).

Thereafter, further purification is performed by treatment in a gaseousatmosphere comprising a halogen or a compound thereof under reducedpressure, namely at the pressure between 0.1 and 0.2 MPa (preferablybetween 0.5 Pa and 0.05 MPa), and at the temperature of 2000° C. orhigher (preferably 2050° C. to 2400° C.) to thereby eliminate thosemetals which are capable of forming volatile halides (ultrahighpurification process).

Further, the carbonaceous material performed by these high purificationprocess is heated between 1400 and 1600° C., preferably between 1450 and1550° C., in a vacuum furnace at the reduced pressure 100 Pa or lower(preferably 50 Pa or lower) for a period of 5 hours or longer(preferably 10 hours or longer) to thereby eliminate volatile impuritiessuch as nitrogen and oxygen (degassing (nitrogen elimination) process).

Finally, following the degassing (nitrogen elimination) process,hydrogen is introduced into the vacuum furnace heated at the temperaturebetween 1400 and 1600° C. (preferably between 1450 and 1550° C.) at thepressure between 100 and 1000 Pa (preferably between 200 and 900 Pa) tothereby eliminate those impurities capable of readily forming volatilehydrides and to hydrogenate the surface of the carbonaceous material sothat such impurities as nitrogen (N), oxygen (O), phosphorus (P) andsulfur (S) can hardly adsorb to the treated surface upon release thereofto the atmosphere (hydrogenation process).

Those treatments make it possible to remove those impurities occurringin pores of the carbonaceous material and/or chemically bound to carbonatoms thereof and, at the same time, prevent the impurities fromadsorption thereto again.

An example of such process for producing the high purity carbonaceousmaterial of the invention is given below.

1. High Purification Process

The carbonaceous material to be treated is placed in a graphitizationfurnace heated between 2400 and 2800° C. at atmospheric pressure, anddichlorodifluoromethane is introduced in the furnace. By this treatment,boron (B) and vanadium (V) can be eliminated with effectively.

2. Ultrahigh Purification Process

The carbonaceous material is put in a vacuum furnace heated between2000° C. and 2400° C., and chlorine (Cl₂) and dichlorodifluoromethaneare flowed into the furnace at the pressure between 10000 Pa and 50000Pa. The flow rate may depend on the amount of the material to betreated. A standard flow rate is generally between 0.1 and 1 NLM/kg. Inthis process, metal impurities are mainly eliminated in the main.

3. Degassing (Nitrogen Elimination) Process

The carbonaceous material is put in a vacuum furnace at a reducedpressure of 100 Pa or lower and heated between 1400° C. and 1600° C. for10˜50 hours. Volatile impurities such as nitrogen and oxygen areeliminated primarily.

4. Hydrogenation Treatment Process

The carbonaceous material is kept in the vacuum furnace heated between1400° C. and 1600° C. for 1 to 10 hours while introducing hydrogen atthe pressure between 100 Pa and 1000 Pa. Those impurities, which readilyform volatile hydrides, are removed and hydrogen is allowed to beadsorbed on the surface of the carbonaceous material under treatment tothereby prevent such impurities as nitrogen (N), oxygen (O), phosphorus(P) and sulfur (S) from adsorbing to the carbonaceous material again.

The analytical method SIMS (secondary ion mass spectrometry) isdescribed here.

SIMS is a method of atomic compositional analysis which measures mass ofsputtered charged particle from the material surface with primary ions(generally O₂ ⁺, Cs⁺ or Ga⁺ ions) accelerated to several hundred voltsto 20 kV. The most characteristic feature of SIMS is that all theelements, from ¹H to ²³⁸U, contained in the material can be detected.SIMS is classified to static SIMS and dynamic SIMS according to thequantity of primary ions used for sputtering. The latter, namely dynamicSIMS, was used in evaluating the effects of the invention.

The SIMS used for determining impurity concentrations in the high puritycarbonaceous material of the invention was CAMECA IMS·3f·4f·4.5f.Different primary ion species were used according to the elements to bemeasured. Thus, O₂ ⁺ ions were used as primary ions for boron (B),aluminum (Al), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe) andnickel (Ni), and Cs⁺ ions were used for nitrogen (N), oxygen (O),fluorine (F), phosphorus (P), sulfur (S) and chlorine (Cl). Afteretching to a depth of 5˜10 μm using such primary ions, the value at thetime when the concentration of an element became constant was taken asthe concentration of that element.

For the measurements, test specimens, 7 mm×7 mm×1 mm in size, wereprepared in advance from each of the samples of the carbonaceousmaterials and the ceramic coated carbonaceous materials produced bycoating on the high purity carbon material as substrate by the thermalCVD method.

EXAMPLE 1

First, a carbonaceous material to serve as the base material of the highpurity carbonaceous material of the invention was prepared using anordinary pressure: graphitization and high purification attainingfurnace.

The heating elements in the atmospheric pressure graphitization and highpurification attaining furnace were gradually heated and then agraphitized, isotropic carbon material, 20 mm×20 mm×2 mm in size,produced by Toyo Tanso Co., Ltd. was heated at 2450° C. and at 1 atm.Halogen gases or a halogen compound gases, for exampledichlorodifluoromethane, was fed to the furnace for about 8 hours (forexample, at a flow rate of about 1˜7 NLM, although the flow rate may bevaried according to the amount of carbonaceous material placed in thefurnace as vacuum vessel) (high purity process).

The high purity carbonaceous material obtained in the high purificationprocess was then maintained at 2250° C. in the furnace under reducedpressure, and halogen gases or halogen compound gases, for exampledichlorodifluoromethane, was fed again to the furnace. This treatmentwas carried out for 5 hours while maintaining the furnace pressure at1000 Pa (ultrahigh purity process).

Thereafter, the material was cooled to 1450° C. at the pressure of 10 Paand then maintained at 1450° C. for 48 hours (degassing (nitrogenelimination) process).

After the nitrogen gas elimination process, the material was maintainedat 100 Pa for 1 hour with hydrogen introducing (hydrogenation process).

Then, argon gas, as a rare gas, was introduced into the furnace and thematerial was cooled to room temperature. After cooling to roomtemperature, the material, together was sealed with argon gas in apolyethylene resin film bag and stored so that it might not be exposedto the air.

EXAMPLE 2

A graphite material was treated in the same high purification processand ultrahigh purification process as in Example 1 and then once takenout of the furnace. It was sealed in a polyethylene resin film bag, andstored with argon gas so that it might not be exposed to the air. Thisgraphite material was taken out of the polyethylene resin film bag andput again in the furnace and heated again to 1450° C. The furnace insidepressure was reduced to 10 Pa, then heat treatment was carried out for48 hours (degassing (nitrogen elimination) process). After thepredetermined period of heat treatment, the material was maintained at100 Pa for 1 hour while introducing hydrogen into the furnace(hydrogenation process). Argon gas, as a rare gas, was introduced intothe furnace, and the material was cooled to room temperature. Aftercooling to room temperature, the material, together was sealed withargon gas in a polyethylene resin film bag and stored so that it mightnot be exposed to the air.

EXAMPLE 3

The procedure of Example 1 was carried out in the same manner exceptthat, after reducing the vessel inside pressure to 10⁻² Pa, thedegassing (nitrogen elimination) process was carried out at 1450° C. for24 hours and the hydrogenation process was then carried out at 1450° C.

EXAMPLE 4

A base material having the same size as in Example 1 was prepared from aC/C material (carbon fiber-reinforced carbon composite material)produced by Toyo Tanso Co., Ltd. and treated in the same manner as inExample 1.

EXAMPLE 5

A base material, 20 mm×20 mm×1 mm in size, was prepared from anexfoliated graphite sheet material produced by Toyo Tanso Co., Ltd. andtreated in the same manner as in Example 1.

EXAMPLE 6

The same sample base material as used in Example 1 was treated in thesame manner as in Example 1 except that the graphite material after thehigh purification process was treated in the ultrahigh purificationprocess at 2100° C. for 5 hours and, thereafter, the degassing (nitrogenelimination) process was carried out at 1400° C. for 20 hours and thehydrogenation process was carried out at 1400° C. and at 100 Pa for 1hour while introducing hydrogen. The material thus obtained was used asthe sample of Example 6.

EXAMPLE 7

The same sample base material as used in Example 1 was treated in thesame manner as in Example 1 except that the graphite material after thehigh purification process was treated in the ultrahigh purificationprocess at 2100° C. for 5 hours and, thereafter, the degassing (nitrogenelimination) process was carried out at 1500° C. for 20 hours and thehydrogenation process was carried out at 1500° C. and at 100 Pa for 1hour while introducing hydrogen. The material thus obtained was used asthe sample of Example 7.

EXAMPLE 8

The same graphite material as that obtained in Example 1 through thehigh purification and ultrahigh purification processes, degassing(nitrogen elimination) process and hydrogenation process was used as asubstrate and coated with 100 μm thickness of SiC by the thermal CVDmethod. The material thus obtained was used as the sample of Example 8.

COMPARATIVE EXAMPLE 1

The same sample base material was treated in the same manner as inExample 1 except that the graphite material after the high purificationprocess was subjected neither to the ultrahigh purification process northe degassing (nitrogen elimination) process but was cooled withnitrogen gas and stored in the atmosphere. The resulting material wasused as the sample of Comparative Example 1.

COMPARATIVE EXAMPLE 2

The graphite material after completion of the ultrahigh purificationprocess alone was not subjected to the degassing (nitrogen elimination)process but was cooled with nitrogen gas and stored in the atmosphere.The resulting material was used as the sample of Comparative Example 2.

COMPARATIVE EXAMPLE 3

The graphite material after completion of the high purification andultrahigh purification processes carried out in the same manner as inExample 1 was not subjected to the degassing (nitrogen elimination)process but was cooled with nitrogen gas and stored in the atmosphere.The resulting material was used as the sample of Comparative Example 3.

COMPARATIVE EXAMPLE 4

The procedure of Example 1 was followed in the same manner except that,after reduction of the furnace pressure to 10 Pa, the degassing(nitrogen elimination) process was carried out at 1450° C. for 48 hoursand that the hydrogenation process was not carried out. Thethus-obtained material was used as the sample of Comparative Example 4.

COMPARATIVE EXAMPLE 5

The procedure of Example 1 was followed in the same manner except that,after vacuuming pressure to 10 Pa, the degassing (nitrogen elimination)process was carried out at 1300° C. for 48 hours and that thehydrogenation process was carried out at 1300° C., at 100 Pa for 1 hourwhile introducing hydrogen. The thus-obtained material was used as thesample of Comparative Example 5.

(COMPARATIVE EXAMPLE 6

The same graphite material as used in Example 1 was subjected to theultrahigh purification process, degassing (nitrogen elimination) processand hydrogenation process in the same manner without carrying out thehigh purification process. The thus-obtained material was used as thesample of Comparative Example 6.

COMPARATIVE EXAMPLE 7

The procedure of Example 1 was followed in the same manner except that,after vacuuming pressure to 10 Pa, the degassing (nitrogen elimination)process was carried out at 1200° C. for 48 hours. The thus-obtainedmaterial was used as the sample of Comparative Example 7.

COMPARATIVE EXAMPLE 8

The same C/C material (product of Toyo Tanso Co., Ltd.) as used inExample 4 was treated in the same manner as in Comparative Example 1.The thus-obtained material was used as the sample of Comparative Example8.

COMPARATIVE EXAMPLE 9

The same exfoliated graphite sheet material (product of Toyo Tanso Co.,Ltd.) as used in Example 5 was treated in the same manner as inComparative Example 1. The thus-obtained material was used as the sampleof Comparative Example 9.

COMPARATIVE EXAMPLE 10

On the surface of the graphite material obtained after completion of thehigh purification and ultrahigh purification processes in the samemanner as in Comparative Example 3, there was coated SiC layer in thesame manner as in Example 8. The thus-obtained material was used as thesample of Comparative Example 10.

The impurity concentrations in the graphite materials of Examples 1 to 7and Comparative Examples 1 to 9 were determined by the SIMS methoddescribed hereinabove. The impurity concentrations in the samples ofExamples 1 to 7 are summarized in Table 1, and the impurityconcentrations in the samples of Comparative Examples 1 to 9 aresummarized in Table 2. The impurity concentrations in the sample ofExample 8 and the sample of Comparative Example 10 were also determinedby the SIMS method described above. The impurity concentrations in thesamples of Example 8 and Comparative Example 10 are summarized in Table3. TABLE 1 (unit: atoms/cm³) Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 B   1.5 × 10¹⁵   4.0 × 10¹⁵   4.2 × 10¹⁴  9.8 × 10¹⁵   4.2 × 10¹⁶   9.9 × 10¹⁵   9.7 × 10¹⁵ N   5.0 × 10¹⁸   6.0× 10¹⁶ <4.0 × 10¹⁶   8.9 × 10¹⁵   4.2 × 10¹⁷   2.1 × 10¹⁶   4.3 × 10¹⁶ O  9.5 × 10¹⁶   1.2 × 10¹⁷   8.3 × 10¹⁵   2.8 × 10¹⁷   9.7 × 10¹⁷   1.3 ×10¹⁸   3.0 × 10¹⁷ F   2.2 × 10¹⁴   4.0 × 10¹⁴   2.0 × 10¹⁴   3.0 × 10¹⁴  2.5 × 10¹⁴   2.5 × 10¹⁴   2.5 × 10¹⁴ Al <4.0 × 10¹³ <4.0 × 10¹³ <4.0 ×10¹³ <4.0 × 10¹³ <4.0 × 10¹³   5.3 × 10¹³ <4.0 × 10¹³ P   2.8 × 10¹⁵  1.7 × 10¹⁵   3.2 × 10¹⁴   2.5 × 10¹⁵   8.6 × 10¹⁵   1.6 × 10¹⁶   9.2 ×10¹⁵ S   3.0 × 10¹⁶   2.2 × 10¹⁵   6.5 × 10¹⁴   6.7 × 10¹⁵   9.8 × 10¹⁵  2.1 × 10¹⁵   1.1 × 10¹⁸ Cl <5.5 × 10¹⁴   1.2 × 10¹⁴ <5.5 × 10¹⁴   3.7× 10¹⁵   8.2 × 10¹⁵   5.2 × 10¹⁵   1.3 × 10¹⁵ Ti <5.0 × 10¹³   1.0 ×10¹⁴ <5.0 × 10¹³ <5.0 × 10¹³ <5.0 × 10¹³ <1.1 × 10¹⁴ <1.1 × 10¹⁴ V   4.5× 10¹⁴   4.5 × 10¹⁴   4.0 × 10¹⁴   4.1 × 10¹⁴   7.2 × 10¹⁴   3.2 × 10¹⁵  9.8 × 10¹⁴ Cr   3.8 × 10¹⁴   3.0 × 10¹⁴   2.0 × 10¹⁴   1.8 × 10¹⁴  8.2 × 10¹⁴   4.5 × 10¹⁴   5.2 × 10¹⁴ Fe   4.1 × 10¹⁴   3.6 × 10¹⁴  2.8 × 10¹⁴   5.2 × 10¹⁴   6.5 × 10¹⁴   4.2 × 10¹⁴   8.3 × 10¹⁴ Ni <3.0× 10¹³ <3.0 × 10¹³ <3.0 × 10¹³ <3.0 × 10¹³ <3.0 × 10¹³ <3.0 × 10¹³ <3.0× 10¹³

TABLE 2 (unit: atoms/cm³) Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Comp. Ex.4 Comp. Ex. 5 Comp. Ex. 6 Comp. Ex. 7 Comp. Ex. 8 Comp. Ex. 9 B 1.8 ×10¹⁸ 2.0 × 10¹⁸   1.5 × 10¹⁶   2.1 × 10¹⁵   1.1 × 10¹⁶   2.1 × 10¹⁶  2.5 × 10¹⁸   1.3 × 10¹⁶   7.2 × 10¹⁶ N 1.0 × 10¹⁸ 6.5 × 10¹⁷   4.0 ×10¹⁷   1.3 × 10¹⁷   3.1 × 10¹⁷   1.1 × 10¹⁶   4.8 × 10¹⁷   1.5 × 10¹⁸  9.2 × 10¹⁷ O 3.2 × 10¹⁸ 1.5 × 10¹⁸   1.2 × 10¹⁸   2.1 × 10¹⁸   1.3 ×10¹⁸   1.1 × 10¹⁸   1.1 × 10¹⁸   1.6 × 10¹⁸   1.6 × 10¹⁸ F 7.2 × 10¹⁷4.5 × 10¹⁷   4.0 × 10¹⁴   8.2 × 10¹⁵   4.2 × 10¹⁶   5.4 × 10¹⁶   8.8 ×10¹⁶   1.3 × 10¹⁷   6.5 × 10¹⁵ Al 1.3 × 10¹⁸ 1.5 × 10¹⁵   8.3 × 10¹⁴<4.0 × 10¹³ <4.0 × 10¹³   2.3 × 10¹⁶ <4.0 × 10¹³   7.2 × 10¹³   8.6 ×10¹³ P 7.3 × 10¹⁸ 2.2 × 10¹⁶   1.5 × 10¹⁶   1.7 × 10¹⁶   1.1 × 10¹⁶  4.8 × 10¹⁶   2.0 × 10¹⁶   1.5 × 10¹⁸   1.3 × 10¹⁵ S 6.4 × 10¹⁷ 1.5 ×10¹⁵   6.0 × 10¹⁵   4.2 × 10¹⁵   1.2 × 10¹⁵   7.4 × 10¹⁵   8.2 × 10¹⁶  5.3 × 10¹⁵   2.1 × 10¹⁵ Cl 7.6 × 10¹⁶ 2.5 × 10¹⁶   1.2 × 10¹⁵   8.4 ×10¹⁴   1.5 × 10¹⁵   1.3 × 10¹⁵   1.0 × 10¹⁶   3.8 × 10¹⁵   1.2 × 10¹⁵ Ti7.2 × 10¹⁴ 1.3 × 10¹⁵ <1.1 × 10¹⁵ <5.0 × 10¹³ <5.0 × 10¹³ <5.0 × 10¹³<5.0 × 10¹³ <5.0 × 10¹³ <5.0 × 10¹³ V 8.2 × 10¹⁵ 8.5 × 10¹⁶   1.1 × 10¹⁵  6.2 × 10¹⁴   5.2 × 10¹⁴   2.3 × 10¹⁵   5.8 × 10¹⁴   8.9 × 10¹⁴   1.6 ×10¹⁶ Cr 2.4 × 10¹⁵ 2.0 × 10¹⁵   2.0 × 10¹⁵   5.5 × 10¹⁴   4.8 × 10¹⁴  8.5 × 10¹⁴   4.0 × 10¹⁴   6.2 × 10¹⁴   1.8 × 10¹⁶ Fe 7.5 × 10¹⁴ 6.5 ×10¹⁵   8.3 × 10¹⁴   5.2 × 10¹⁴   6.0 × 10¹⁴   9.8 × 10¹⁴   6.2 × 10¹⁴8.8 × 10¹⁴   1.8 × 10¹⁶ Ni 6.3 × 10¹⁴ 1.5 × 10¹⁵   8.7 × 10¹³ <3.0 ×10¹³ <3.0 × 10¹³   4.8 × 10¹⁴ <3.0 × 10¹³ <3.0 × 10¹³   9.2 × 10¹³

TABLE 3 (unit: atoms/cm³) Example 8 Comparative Example 10 B   4.6 ×10¹⁵   3.5 × 10¹⁷ N   8.7 × 10¹⁶   4.9 × 10¹⁶ Al <6.0 × 10¹³   1.4 ×10¹⁴ Ti <2.5 × 10¹³ <2.5 × 10¹³ V   8.3 × 10¹³   9.9 × 10¹⁴ Fe <3.2 ×10¹³   2.9 × 10¹⁴ Ni   4.7 × 10¹³   8.9 × 10¹⁴

The data in Table 1 and Table 2 indicate that the graphite materialsobtained in Examples 1 to 7 through the nitrogen gas elimination processare lower in nitrogen content as compared with the graphite materials ofComparative Examples 1 to 9 respectively. Accordingly, thelow-nitrogen-concentration graphite materials of Examples 1 to 7 can beused as tools or jigs in the production of SiC semiconductors and soforth while preventing the occurrence of crystal defects insemiconductor devices such as SiC semiconductors.

Furthermore, the use of the graphite materials of Examples 1 to 4 and 7,which are low in boron concentration as well, makes it possible toproduce low-donor-density SiC semiconductors.

The graphite material of Example 6, which has a reduced boronconcentration as well, can be utilized as a tools or jigs for siliconsingle crystals in the Czochralski process, for instance, and can thuscontribute toward markedly reducing the boron concentration in theproduct silicon single crystals.

Further, when the graphite materials of Examples 1 to 7 are used asgraphite parts to be used in nuclear reactors, for example graphitemoderators in nuclear reactors or graphite-made fuel blocks inhigh-temperature gas-cooled reactors, radioactivity can be suppressedowing to their low impurity concentrations.

The data in Table 3 indicate that when the graphite material low inimpurity concentrations is used as the base graphite material for SiCcoating, the concentrations of impurities, for example boron andnitrogen, in addition to metal impurities, in the SiC layer can bereduced. Accordingly, by using the SiC-coated graphite material ofExample 8 as tools or jigs in the manufacture of silicon semiconductors,it becomes possible to reduce the impurity concentrations in epitaxialsilicon layers.

Various alterations and modifications to the present invention can bemade without departing the spirit or scope of the invention claimed inthe appended claims and the above examples are by no means limitative ofthe scope of the invention.

1. A high purity carbonaceous material having an oxygen content of1×10¹⁸ atoms/cm³ or less as determined by SIMS.
 2. A high puritycarbonaceous material according to claim 1 which has a chlorine contentof 1×10¹⁶ atoms/cm³ or less as determined by SIMS.
 3. A high puritycarbonaceous material according to claim 1 or 2 which has a nitrogencontent of 5×10¹⁸ atoms/cm³ or less as determined by SIMS.
 4. A highpurity carbonaceous material according to any of claims 1 to 3 which hasa phosphorus content of 1×10¹⁶ atoms/cm³ or less as determined by SIMS.5. A high purity carbonaceous material according to any of claims 1 to 4which has a sulfur content of not higher than 1×10¹⁶ atoms/cm³ asdetermined by SIMS.
 6. A high purity carbonaceous material according toany of claims 1 to 5 which has a boron content of 5×10¹⁶ atoms/cm³ orless as determined by SIMS.
 7. A high purity carbonaceous materialhaving a nitrogen content of not higher than 1×10¹⁶ atoms/cm³ asdetermined by SIMS.
 8. A high. purity carbonaceous material according toclaim 7 which has a nitrogen content of 5×10¹⁸ atoms/cm³ or less asdetermined by SIMS.
 9. A high purity carbonaceous material according toany of claims 1 to 8 which is intended to be used in the production ofsilicon carbide single crystals, silicon single crystals, galliumnitride single crystals or calcium fluoride single crystals.
 10. A highpurity carbonaceous material according to any of claims 1 to 8 which isintended to be used as tools or utensils in epitaxial growing of siliconcarbide, gallium nitride or silicon.
 11. A high purity carbonaceousmaterial according to any of claims 1 to 8 which is intended to be usedas a substrate for coating with a ceramic layer.
 12. A ceramiclayer-coated high purity carbonaceous material comprising a high puritycarbonaceous material according to any of claims 1 to 8 as a substrateof ceramics coating.